1. Field of the Invention
The present invention relates in general to Differential Phase Shift Keying (D-PSK) demodulation. More particularly, the invention is directed to methods and apparatus providing optimal D-PSK demodulation based on correlation angle distribution.
2. Related Art
The need for high speed robust communications systems has grown dramatically in recent years. Such a demand has been fueled by the need to support various communications market segments, e.g., ever increasing numbers of voice calls, higher information transfer rates, better connectivity to the Internet. Both consumer and business market segments have witnessed unparalleled increases in growth, and such growth is predicted to continue for the foreseeable future. In theory, communications systems could accommodate the demand for increased data throughput by using larger bandwidth for communication. However, bandwidth is a limited resource and is usually highly government regulated. Accordingly, communication systems designers have sought to achieve greater data throughput utilizing existing bandwidth communication channels, either by using more efficient modulation/demodulation schemes, or by finding ways to overcome practical limitations imposed by the communications environment, e.g. the communications channel.
The D-PSK, and particularly Differential Quadrature Phase Shift Keying (D-QPSK) modulation technique, is commonly used in modern wireless communication systems.
PSK modulation techniques encode information bits, such as, for example, “1” and “0” as a phase with respect to some reference. Each pattern of bits is represented by a symbol of a particular phase. A bit is represented by either “0” or “1”.
Alternatively, instead of using the bit patterns to set the phase of the symbol, it can instead be used to control a phase difference of symbols from one to the next. In other words, a particular bit pattern specifies that the symbol has a certain phase difference with respect to the immediately proceeding symbol. The demodulator then determines the changes in phase from one symbol to the next received symbol rather than determining the absolute phase of a symbol itself. Since this scheme depends on the difference in phase between successive symbols, rather than a phase with respect to a reference symbol, it is referred to as differential PSK (D-PSK). For example, for a D-QPSK, a phase difference π/4 between two successive symbols represents the bit pattern (0,0), a phase difference 3π/4 represents (1,0), −3π/4 represents (1,1) and −π/4 represents (0,1).
Because of the imperfection of communication channels and signal processing carried out by a receiver's front end, two successively received symbols rarely have a measurable angular difference that is exactly equal to one of the angular differences of a D-PSK modulation table. Communication channels are often dynamic in that they introduce amplitude and phase distortions, as well as noise contributions, into a propagating signal in an irregular and time-varying manner. When symbols are received, the angular differences between successive symbols must be interpreted by a demodulator to determine which bit patterns they represent.
In order to improve the performance of a given communications system, it is desirable to utilize a D-PSK demodulation scheme that is robust to dynamic channels, imperfect synchronization and other imperfect front end receiving processes.
The conventional approach to D-PSK demodulation is a non-coherent approach. Two successively received symbols are first correlated to determine a phase difference between them. Standard PSK demodulation is then carried out to estimate the bit pattern represented by the determined phase difference. Standard PSK is optimal when a communication channel conforms to the assumption that successively received symbols are subject to the same unknown phase.
In the real world, however, communication channels in many situations are not static, but rather, dynamic. Hence the phase rotations in received symbols, which result from dynamic channel conditions and imperfect front end receiving operations, are time-varying. Therefore, the performance of a conventional D-PSK demodulation system optimized for a static communications channel can be considerably degraded in such circumstances. For example, a conventional D-QPSK demodulation algorithm in an ISDB-T compliant system, without symbol interleaving, provides a bit error rate (BER) of 0.002 before Reed-Solomon decoding for a 2-path Rayleigh fading channel with a Doppler frequency of 70 Hz.
Based on the basic conventional approach, several variations on conventional D-PSK demodulation have been proposed and can be found in the literature, such as, for example, [Divsalar1990], [Hewavithana2003] and [Wong1992] which assumed different channel characteristics and exploited different optimization criteria. The complete citations to these references are: [Divsalar 90] D. Divsalar and M. Simon, “Multiple-symbol differential detection of MPSK,” IEEE Trans. Communication, vol. 38, no. 3, March 1990, pp. 300-308; [Wong1992] P. C. Wong and P. T. Mathiopoulos, “Nonredundant error correction analysis and evaluation of differential detected pi/4-shift DQPSK systems in a combined CCI and AWGN environment,” IEEE Trans. Vehicular Technology, vol. 41, no. 1 Feb. 1992, pp. 35-48; and [Hewavithana2003] T. C. Hewavithana and M. Brookes, “Soft decisions for DQPSK demodulation for the Viterbi decoding of the convolutional codes,” Proceedings of 2003 International Conference on Acoustic, Speech and Signal Processing, pp. IV-17-IV-20.
As described more fully below, this invention is directed to a different type of variation than those described in the literature.